Enzymatic Hydrolysis of Polyester Thin Films: Real-Time Analysis of Film Mass Changes and Dissipation Dynamics

Functional characterization and structural insights of three cutinases from the ascomycete Fusarium verticillioides

Cutinases are serine esterases that belong to the α/β hydrolases superfamily. The natural substrates for these enzymes are cutin and suberin, components of the plant cuticle, the first barrier in the defense system against pathogen invasion. It is well-reported that plant pathogens produce cutinases to facilitate infection. Fusarium verticillioides, one important corn pathogens, is an ascomycete upon which its cutinases are poorly explored. Consequently, the objective of this study was to perform the biochemical characterization of three precursor cutinases (FvCut1, FvCut2, and FvCut3) from F. verticillioides and to obtain structural insights about them. The cutinases were produced in Escherichia coli and purified. FvCut1, FvCut2, and FvCut3 presented optimal temperatures of 20, 40, and 35 °C, and optimal pH of 9, 7, and 8, respectively. Some chemicals stimulated the enzymatic activity. The kinetic parameters revealed that FvCut1 has higher catalytic efficiency (Kcat/Km) in the p-nitrophenyl-butyrate (p-NPB) substrate. Nevertheless, the enzymes were not able to hydrolyze polyethylene terephthalate (PET). Furthermore, the three-dimensional models of these enzymes showed structural differences among them, mainly FvCut1, which presented a narrower opening cleft to access the catalytic site. Therefore, our study contributes to exploring the diversity of fungal cutinases and their potential biotechnological applications.

Study on Properties and Degradation Behavior of Poly (Adipic Acid/Butylene Terephthalate-Co-Glycolic Acid) Copolyester Synthesized by Quaternary Copolymerization

At present, the development and usage of degradable plastics instead of traditional plastics is an effective way to solve the pollution of marine microplastics. Poly (butylene adipate-co-terephthalate) (PBAT) is known as one of the most promising biodegradable materials. Nevertheless, the degradation rate of PBAT in water environment is slow. In this work, we successfully prepared four kinds of high molecular weight polyester copolyesters (PBATGA) via quaternary copolymerization. The results showed that the intrinsic viscosity of PBATGA copolymers ranged from 0.74 to 1.01 dL/g with a glycolic acid content of 0–40%. PBATGA copolymers had excellent flexibility and thermal stability. The tensile strength was 5~40 MPa, the elongation at break was greater than 460%, especially the elongation at break of PBATGA10 at 1235%, and the thermal decomposition temperature of PBATGA copolyesters was higher than 375 °C. It was found that PBATGA copolyester had a faster hydrolysis rate than PBAT, and the weight loss of PBATGA copolymers showed a tendency of pH = 12 > Lipase ≈ pH = 7 > pH = 2. The quaternary polymerization of PBAT will have the advantage of achieving industrialization, unlike the previous polymerization process. In addition, the polymerization of PBATGA copolyesters not only utilizes the by-products of the coal chemical industry, but also it can be promising in the production of biodegradable packaging to reduce marine plastic pollution.

Sustainable Design of Structural and Functional Polymers for a Circular Economy

To achieve a sustainable circular economy, polymer production must start transitioning to recycled and biobased feedstock and accomplish CO₂ emission neutrality. This is not only true for structural polymers, such as in packaging or engineering applications, but also for functional polymers in liquid formulations, such as adhesives, lubricants, thickeners or dispersants. At their end of life, polymers must be either collected and recycled via a technical pathway, or be biodegradable if they are not collectable. Advances in polymer chemistry and applications, aided by computational material science, open the way to addressing these issues comprehensively by designing for recyclability and biodegradability. This Review explores how scientific progress, together with emerging regulatory frameworks, societal expectations and economic boundary conditions, paint pathways for the transformation towards a circular economy of polymers.

Biodegradation of Biodegradable Polymers in Mesophilic Aerobic Environments

Finding alternatives to diminish plastic pollution has become one of the main challenges of modern life. A few alternatives have gained potential for a shift toward a more circular and sustainable relationship with plastics. Biodegradable polymers derived from bio- and fossil-based sources have emerged as one feasible alternative to overcome inconveniences associated with the use and disposal of non-biodegradable polymers. The biodegradation process depends on the environment's factors, microorganisms and associated enzymes, and the polymer properties, resulting in a plethora of parameters that create a complex process whereby biodegradation times and rates can vary immensely. This review aims to provide a background and a comprehensive, systematic, and critical overview of this complex process with a special focus on the mesophilic range. Activity toward depolymerization by extracellular enzymes, biofilm effect on the dynamic of the degradation process, CO2 evolution evaluating the extent of biodegradation, and metabolic pathways are discussed. Remarks and perspectives for potential future research are provided with a focus on the current knowledge gaps if the goal is to minimize the persistence of plastics across environments. Innovative approaches such as the addition of specific compounds to trigger depolymerization under particular conditions, biostimulation, bioaugmentation, and the addition of natural and/or modified enzymes are state-of-the-art methods that need faster development. Furthermore, methods must be connected to standards and techniques that fully track the biodegradation process. More transdisciplinary research within areas of polymer chemistry/processing and microbiology/biochemistry is needed.

Biodegradation of poly(butylene succinate) in soil laboratory incubations assessed by stable carbon isotope labelling

Using biodegradable instead of conventional plastics in agricultural applications promises to help overcome plastic pollution of agricultural soils. However, analytical limitations impede our understanding of plastic biodegradation in soils. Utilizing stable carbon isotope (¹³C-)labelled poly(butylene succinate) (PBS), a synthetic polyester, we herein present an analytical approach to continuously quantify PBS mineralization to ¹³CO₂ during soil incubations and, thereafter, to determine non-mineralized PBS-derived ¹³C remaining in the soil. We demonstrate extensive PBS mineralization (65 % of added ¹³C) and a closed mass balance on PBS−¹³C over 425 days of incubation. Extraction of residual PBS from soils combined with kinetic modeling of the biodegradation data and results from monomer (i.e., butanediol and succinate) mineralization experiments suggest that PBS hydrolytic breakdown controlled the overall PBS biodegradation rate. Beyond PBS biodegradation in soil, the presented methodology is broadly applicable to investigate biodegradation of other biodegradable polymers in various receiving environments.

This study applies stable carbon isotope labelling to study polymer biodegradation in soils. This labelling enables accurate and precise tracking of polymer carbon during biodegradation and, thereby, provides a holistic picture of this process.

Mechanism-Based Design of Efficient PET Hydrolases

Polyethylene terephthalate (PET) is the most widespread synthetic polyester, having been utilized in textile fibers and packaging materials for beverages and food, contributing considerably to the global solid waste stream and environmental plastic pollution. While enzymatic PET recycling and upcycling have recently emerged as viable disposal methods for a circular plastic economy, only a handful of benchmark enzymes have been thoroughly described and subjected to protein engineering for improved properties over the last 16 years. By analyzing the specific material properties of PET and the reaction mechanisms in the context of interfacial biocatalysis, this Perspective identifies several limitations in current enzymatic PET degradation approaches. Unbalanced enzyme-substrate interactions, limited thermostability, and low catalytic efficiency at elevated reaction temperatures, and inhibition caused by oligomeric degradation intermediates still hamper industrial applications that require high catalytic efficiency. To overcome these limitations, successful protein engineering studies using innovative experimental and computational approaches have been published extensively in recent years in this thriving research field and are summarized and discussed in detail here. The acquired knowledge and experience will be applied in the near future to address plastic waste contributed by other mass-produced polymer types (e.g., polyamides and polyurethanes) that should also be properly disposed by biotechnological approaches.

Microplastic contaminants in the aqueous environment, fate, toxicity consequences, and remediation strategies

Microplastic is a fragmented plastic part that emerges as a potential marine and terrestrial contaminant. The microplastic wastes in marine and soil environments cause severe problems in living systems. Microplastic wastes have been linked to various health problems, including reproductive harm and obesity, plus issues such as organ problems and developmental delays in children. Recycling plastic/microplastics from the environment is very low, so remediating these polymers after their utilization is of paramount concern. The microplastic causes severe toxic effects and contaminates the environment. Microplastic affects marine life, microorganism in soil, soil enzymes, plants system, and physicochemical properties. Ecotoxicology of the microplastic raised many questions about its use and development from the environment. Various physicochemical and microbial technologies have been developed for their remediation from the environment. The microplastic effects are linked with its concentration, size, and shape in contaminated environments. Microplastic is able to sorb the inorganic and organic contaminants and affect their fate into the contaminated sites. Microbial technology is considered safer for the remediation of the microplastics via its unique metabolic machinery. Bioplastic is regarded as safer and eco-friendly as compared to plastics. The review article explored an in-depth understanding of the microplastic, its fate, toxicity to the environment, and robust remediation strategies.

A comprehensive and critical review on key elements to implement enzymatic PET depolymerization for recycling purposes

Plastics production and recycling chains must be refitted to a circular economy. Poly(ethylene terephthalate) (PET) is especially suitable for recycling because of its hydrolysable ester bonds and high environmental impact due to employment in single-use packaging, so that recycling processes utilizing enzymes are a promising biotechnological route to monomer recovery. However, enzymatic PET depolymerization still faces challenges to become a competitive route at an industrial level. In this review, PET characteristics as a substrate for enzymes are discussed, as well as the analytical methods used to evaluate the reaction progress. A comprehensive view on the biocatalysts used is discussed. Subsequently, different strategies pursued to improve enzymatic PET depolymerization are presented, including enzyme modification through mutagenesis, utilization of multiple enzymes, improvement of the interaction between enzymes and the hydrophobic surface of PET, and various reaction conditions (e.g., particle size, reaction medium, agitation, and additives). All scientific developments regarding these different aspects of PET depolymerization are crucial to offer a scalable and competitive technology. However, they must be integrated into global processes from upstream to downstream, discussed here at the final sections, which must be evaluated for their economic feasibility and life cycle assessment to check if PET recycling chains can be broadly incorporated into the future circular economy.

Enzymatic degradation of polyethylene terephthalate nanoplastics analyzed in real time by isothermal titration calorimetry

Plastics are globally used for a variety of benefits. As a consequence of poor recycling or reuse, improperly disposed plastic waste accumulates in terrestrial and aquatic ecosystems to a considerable extent. Large plastic waste items become fragmented to small particles through mechanical and (photo)chemical processes. Particles with sizes ranging from millimeter (microplastics, <5 mm) to nanometer (nanoplastics, NP, <100 nm) are apparently persistent and have adverse effects on ecosystems and human health. Current research therefore focuses on whether and to what extent microorganisms or enzymes can degrade these NP. In this study, we addressed the question of what information isothermal titration calorimetry, which tracks the heat of reaction of the chain scission of a polyester, can provide about the kinetics and completeness of the degradation process. The majority of the heat represents the cleavage energy of the ester bonds in polymer backbones providing real-time kinetic information. Calorimetry operates even in complex matrices. Using the example of the cutinase-catalyzed degradation of polyethylene terephthalate (PET) nanoparticles, we found that calorimetry (isothermal titration calorimetry-ITC) in combination with thermokinetic models is excellently suited for an in-depth analysis of the degradation processes of NP. For instance, we can separately quantify i) the enthalpy of surface adsorption ∆_AdsH = 129 ± 2 kJ mol^-1, ii) the enthalpy of the cleavage of the ester bonds ∆_EBH = -58 ± 1.9 kJ mol^-1 and the apparent equilibrium constant of the enzyme substrate complex K = 0.046 ± 0.015 g L^-1. It could be determined that the heat production of PET NP degradation depends to 95% on the reaction heat and only to 5% on the adsorption heat. The fact that the percentage of cleaved ester bonds (η = 12.9 ± 2.4%) is quantifiable with the new method is of particular practical importance. The new method promises a quantification of enzymatic and microbial adsorption to NP and their degradation in mimicked real-world aquatic conditions.

Evaluation of surface layer stability of surface-modified polyester biomaterials

Surface modification of biomaterials is a strategy used to improve cellular and in vivo outcomes. However, most studies do not evaluate the lifetime of the introduced surface layer, which is an important aspect affecting how a biomaterial will interact with a cellular environment both in the short and in the long term. This study evaluated the surface layer stability in vitro in buffer solution of materials produced from poly(lactic-co-glycolic acid) (50:50) and polycaprolactone modified by hydrolysis and/or grafting of hydrophilic polymers using grafting from approaches. The data presented in this study highlight the shortcomings of using model substrates (e.g., spun-coated films) rather than disks, particles, and scaffolds. It also illustrates how similar surface modification strategies in some cases result in very different lifetimes of the surface layer, thus emphasizing the need for these studies as analogies cannot always be drawn.

Photochemical Transformation of Poly(butylene adipate- co-terephthalate) and Its Effects on Enzymatic Hydrolyzability

Biodegradable polyesters are being increasingly used to replace conventional, nondegradable polymers in agricultural applications such as plastic film for mulching. For many of these applications, poly(butylene adipate- co-terephthalate) (PBAT) is a promising biodegradable material. However, PBAT is also susceptible to photochemical transformations. To better understand how photochemistry affects the biodegradability of PBAT, we irradiated blown, nonstabilized, transparent PBAT films and studied their enzymatic hydrolysis, which is considered the rate-limiting step in polyester biodegradation. In parallel, we characterized the irradiated PBAT films by dynamic mechanical thermal analysis. The rate of enzymatic PBAT hydrolysis decreased when the density of light-induced cross-links within PBAT exceeded a certain threshold. Mass-spectrometric analysis of the enzymatic hydrolysis products of irradiated PBAT films provided evidence for radical-based cross-linking of two terephthalate units that resulted in the formation of benzophenone-like molecules. In a proof-of-principle experiment, we demonstrated that the addition of photostabilizers to PBAT films mitigated the negative effect of UV irradiation on the enzymatic hydrolyzability of PBAT. This work advances the understanding of light-induced changes on the enzyme-mediated hydrolysis of aliphatic-aromatic polyesters and will therefore have important implications for the development of biodegradable plastics.

Biodegradation of Polymeric Mulch Films in Agricultural Soils: Concepts, Knowledge Gaps, and Future Research Directions

The agricultural use of conventional, polyethylene-based mulch films leads to the accumulation of remnant film pieces in agricultural soils with negative impacts for soil productivity and ecology. A viable strategy to overcome this accumulation is to replace conventional with biodegradable mulch films composed of polymers designed to be degraded by soil microorganisms. However, understanding polymer biodegradation in soils remains a significant challenge due to its dependence on polymer properties, soil characteristics, and prevailing environmental conditions. This perspective aims to advance our understanding of the three fundamental steps underlying biodegradation of mulch films in agricultural soils: colonization of the polymer film surfaces by soil microorganisms, depolymerization of the polymer films by extracellular microbial hydrolases, and subsequent microbial assimilation and utilization of the hydrolysis products for energy production and biomass formation. The perspective synthesizes the current conceptual understanding of these steps and highlights existing knowledge gaps. The discussion addresses future research and analytical advancements required to overcome the knowledge gaps and to identify the key polymer properties and soil characteristics governing mulch film biodegradation in agricultural soils.

Intensified heterotrophic denitrification in constructed wetlands using four solid carbon sources: Denitrification efficiency and bacterial community structure

Biodenitrification using solid carbon sources is a cost-effective way for nitrate removal. In the study, wheat straw, cotton, poly(butylene succinate), and newspaper was chosen as the carbon source to compare the denitrification efficiency and bacterial communities in constructed wetlands. Parameters including COD, NO₃^--N, NO₂^--N and total nitrogen (TN) were analyzed. Results indicated that newspaper provided significantly higher NO₃^--N and TN removal efficiency than the other three solid carbon sources in low-temperature condition. Moreover, both newspaper and wheat straw allowed high NO₃^--N and TN removal efficiency in high-temperature condition. According to pyrosequencing analysis, denitrifying bacteria Dechloromonas and Thauera were the predominant genus in the anaerobic zone of CO- (3.92 and 2.35%, respectively), WS- (1.97 and 1.02%, respectively) and NP-CWs (1.71 and 1.31%, respectively). Genus of Levilinea was enriched in NP- (1.02%) and WS-CWs (0.91%). Furthermore, genus Paludibacter (2.69%) and Saccharofermentans (3.14%) showed high relative abundance in WS-CWs.

Enzymatic degradation of poly(butylene succinate) with different molecular weights by cutinase

Poly(butylene succinate) (PBS) films with different molecular weights were enzymatically degraded by cutinase. Changes in the properties of the films before and after enzymatic degradation were studied through scanning electron microscopy, differential scanning calorimetry, thermogravimetry, X-ray powder diffraction, proton nuclear magnetic resonance, and gel-permeation chromatography analysis. The weight loss of the films initially decreased and then increased with increasing molecular weight. Crystallinity was inversely proportional to weight loss and tended to decrease with prolonged degradation time. Crystalline and amorphous regions were simultaneously degraded. The thermal stability of PBS films decreased after enzymatic degradation. PBS was the main component of the enzymatically degraded polymers. The molecular weights of the films did not considerably change before and after degradation by cutinase.

Enzymatic Hydrolysis of Polyester Thin Films at the Nanoscale: Effects of Polyester Structure and Enzyme Active-Site Accessibility

Biodegradable polyesters have a large potential to replace persistent polymers in numerous applications and to thereby reduce the accumulation of plastics in the environment. Ester hydrolysis by extracellular carboxylesterases is considered the rate-limiting step in polyester biodegradation. In this work, we systematically investigated the effects of polyester and carboxylesterase structure on the hydrolysis of nanometer-thin polyester films using a quartz-crystal microbalance with dissipation monitoring. Hydrolyzability increased with increasing polyester-chain flexibility as evidenced from differences in the hydrolysis rates and extents of aliphatic polyesters varying in the length of their dicarboxylic acid unit and of poly(butylene adipate-co-terephthalate) (PBAT) polyesters varying in their terephthalate-to-adipate ratio by Rhizopus oryzae lipase and Fusarium solani cutinase. Nanoscale nonuniformities in the PBAT films affected enzymatic hydrolysis and were likely caused by domains with elevated terephthalate contents that impaired enzymatic hydrolysis. Yet, the cutinase completely hydrolyzed all PBAT films, including films with a terephthalate-to-adipate molar ratio of one, under environmentally relevant conditions (pH 6, 20 °C). A comparative analysis of the hydrolysis of two model polyesters by eight different carboxylesterases revealed increasing hydrolysis with increasing accessibility of the enzyme active site. Therefore, this work highlights the importance of both polyester and carboxylesterase structure to enzymatic polyester hydrolysis.

Probing the interactions of organic molecules, nanomaterials, and microbes with solid surfaces using quartz crystal microbalances: methodology, advantages, and limitations

Quartz crystal microbalances (QCMs) provide a new analytical opportunity and prospect to characterize many environmental processes at solid/liquid interfaces, thanks to their almost real-time measurement of physicochemical changes on their quartz sensor. This work reviews the applications of QCMs in probing the interactions of organic molecules, nanomaterials (NMs) and microbes with solid surfaces. These interfacial interactions are relevant to critical environmental processes such as biofilm formation, fate and transport of NMs, fouling in engineering systems and antifouling practices. The high sensitivity, real-time monitoring, and simultaneous frequency and dissipation measurements make QCM-D a unique technique that helps reveal the interaction mechanisms for the abovementioned processes (e.g., driving forces, affinity, kinetics, and the interplay between surface chemistry and solution chemistry). On the other hand, QCM measurement is nonselective and spatially-dependent. Thus, caution should be taken during data analysis and interpretation, and it is necessary to cross-validate the results using complementary information from other techniques for more quantitative and accurate interpretation. This review summarizes the general methodologies for collecting and analyzing raw QCM data, as well as for evaluating the associated uncertainties. It serves to help researchers gain deeper insights into the fundamentals and applications of QCMs, and provides new perspectives on future research directions.

Enzymatic Degradation of Aromatic and Aliphatic Polyesters by P. pastoris Expressed Cutinase 1 from Thermobifida cellulosilytica

To study hydrolysis of aromatic and aliphatic polyesters cutinase 1 from Thermobifida cellulosilytica (Thc_Cut1) was expressed in P. pastoris. No significant differences between the expression of native Thc_Cut1 and of two glycosylation site knock out mutants (Thc_Cut1_koAsn and Thc_Cut1_koST) concerning the total extracellular protein concentration and volumetric activity were observed. Hydrolysis of poly(ethylene terephthalate) (PET) was shown for all three enzymes based on quantification of released products by HPLC and similar concentrations of released terephthalic acid (TPA) and mono(2-hydroxyethyl) terephthalate (MHET) were detected for all enzymes. Both tested aliphatic polyesters poly(butylene succinate) (PBS) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were hydrolyzed by Thc_Cut1 and Thc_Cut1_koST, although PBS was hydrolyzed to significantly higher extent than PHBV. These findings were also confirmed via quartz crystal microbalance (QCM) analysis; for PHBV only a small mass change was observed while the mass of PBS thin films decreased by 93% upon enzymatic hydrolysis with Thc_Cut1. Although both enzymes led to similar concentrations of released products upon hydrolysis of PET and PHBV, Thc_Cut1_koST was found to be significantly more active on PBS than the native Thc_Cut1. Hydrolysis of PBS films by Thc_Cut1 and Thc_Cut1_koST was followed by weight loss and scanning electron microscopy (SEM). Within 96 h of hydrolysis up to 92 and 41% of weight loss were detected with Thc_Cut1_koST and Thc_Cut1, respectively. Furthermore, SEM characterization of PBS films clearly showed that enzyme tretment resulted in morphological changes of the film surface.

High-Throughput Analysis of Enzymatic Hydrolysis of Biodegradable Polyesters by Monitoring Cohydrolysis of a Polyester-Embedded Fluorogenic Probe

Biodegradable polyesters have the potential to replace nondegradable, persistent polymers in numerous applications and thereby alleviate plastic accumulation in the environment. Herein, we present an analytical approach to study enzymatic hydrolysis of polyesters, the key step in their overall biodegradation process. The approach is based on embedding fluorescein dilaurate (FDL), a fluorogenic ester substrate, into the polyester matrix and on monitoring the enzymatic cohydrolysis of FDL to fluorescein during enzymatic hydrolysis of the polyester. We validated the approach against established techniques using FDL-containing poly(butylene adipate) films and Fusarium solani cutinase (FsC). Implemented on a microplate reader platform, the FDL-based approach enabled sensitive and high-throughput analysis of the enzymatic hydrolysis of eight aliphatic polyesters by two fungal esterases (FsC and Rhizopus oryzae lipase) at different temperatures. While hydrolysis rates for both enzymes increased with decreasing differences between the polyester melting temperatures and the experimental temperatures, this trend was more pronounced for the lipase than the cutinase. These trends in rates could be ascribed to a combination of temperature-dependent polyester chain flexibility and accessibility of the enzyme active site. The work highlights the capability of the FDL-based approach to be utilized in both screening and mechanistic studies of enzymatic polyester hydrolysis.